A number of industrially employed processes are known for the treatment of gaseous mixtures such as defined above and whose main examples are represented by the various natural gases, which comprise a decarbonation operation, that is a CO.sub.2 removal, and a gasoline stripping operation, that is a separation of the heavy hydrocarbons, for example C.sub.3 and higher, from the gaseous mixture and allowing the said gaseous mixture to be fractionated into the three components referred to above.
These decarbonation and gasoline stripping operations are generally performed separately and form part of a series of operations performed on the gaseous mixture to be treated and comprising chiefly a removal of the CO.sub.2 acidic gas, a drying operation, a water adsorption on a suitable solid such as a molecular sieve, a separation by cryogenic distillation between -30.degree. C. and -90.degree. C. coupled or otherwise with an extraction with a solvent in order to obtain the liquid cut of natural gas and, lastly, heating the treated gas to room temperature, generally in order to feed a commercial gas grid.
In such a scheme of treatment of the gaseous mixture of the natural gas type, containing the abovementioned constituents, lowering of the temperature of the gaseous mixture is made necessary only by the production of the liquid cut of natural gas, no other operation being performed at this temperature level.
In this treatment scheme, the serial carrying out of operations which are based on quite different principles and which are conducted at different temperature levels presents considerable disadvantages. There is very little possibility of thermal integration, and this makes the said treatment scheme extremely costly in terms of energy and in terms of capital cost.
There are also known processes for the treatment of gaseous mixtures of the natural gas type, which make it possible to remove the CO.sub.2 present in the gaseous mixture simultaneously with the production of gaseous hydrocarbons and liquid hydrocarbons and typical of which is the process known as the Ryan-Holmes process and described, in particular, by J. Ryan and F. Schaffert in the journal Chemical Engineering Progress, October 1984, pages 53 to 56. In a process of this kind, after having been dehydrated conventionally and then refrigerated, the natural gas to be treated is subjected to a low-temperature distillation carried out in three or four successive stages.
In the three-stage method of operation the dehydrated and refrigerated natural gas is separated, in a first (demethanizer) column into the top of which is injected an additive consisting of a liquid C.sub.4 and higher hydrocarbon fraction, into a gaseous phase containing methane and lighter compounds, and a liquid fraction containing the C.sub.2 and higher hydrocarbons and CO.sub.2. This liquid fraction is separated, in a second (de-ethanizer) column, into which a certain quantity of the additive is also introduced, into a head fraction consisting of CO.sub.2 and a tail fraction containing C.sub.2 and higher hydrocarbons. The tail fraction is then separated, in a third column, into a head fraction consisting of a liquid C.sub.2 -C.sub.4 hydrocarbon fraction and a tail fraction consisting of a liquid C.sub.4 and higher hydrocarbon cut. This cut contains most of the butanes and higher hydrocarbons present in the treated natural gas and from which the appropriate quantity is removed to constitute the additive injected into the first and second columns. The use of this additive prevents the crystallization of CO.sub.2 at the head of the demethanizer and ensures the breaking of the azeotrope which is formed between ethane and CO.sub.2 and facilitates the separation of these compounds in the de-ethanizer. The abovementioned process relies, therefore, essentially on operations of distillation in series.